The neurobehavioral effects of 2,4,5,2',4',5'-hexabromobiphenyl (HBB) and Firemaster FF-1, a commercial mixture of polybrominated biphenyls (PBB), of which HBB is the major component, have been shown to be different when given in amounts equating HBB content. The present study repeats and extends observations of neurobehavioral effects of equal doses of HBB or FF-1. Rats are trained to press a lever in standard operant conditioning chambers to avoid an unsignalled presentation of electric footshock and to escape shock when it is presented (unsignaled continous avoidance) until a stable baseline of responding is achieved. Dosing (10 mg/kg of FF-1, 10 mg/kg of HBB, or corn oil vehicle) is then started. Effects on performance of the avoidance shedule are to be evaluated. Parallel assessment on a screening test battery is to include the following tests: body weight, sensorimotor reflexes and orientation, hind limb strength, forelimb strength, visual placement, inclined screen, spontaneous activity, startle, and rectal temperature. Latency to escape shock in a second apparatus will be tested. Autonomic signs and general health indicators will be noted.